There are several factors to consider in order for an ink to adhere to a substrate. It is generally accepted that the substrate should have a relatively high surface energy compared to the ink. This allows for the rapid laminar flow of the ink over the substrate. An increased surface area (best measured as roughness) is also considered important for the anchorage of the ink to the substrate. And finally, the porous nature of the substrate can aid the absorption of ink for a more intimate bond. Conventional polymeric films usually have an inherent low surface energy (dyne level), tend to have chemically inert, smooth, non-porous surfaces, and thus are non-receptive to bonding with substrates, printing inks, coatings, and adhesives. These conventional polymeric films thus require a print-receptive layer or treatment in order to increase their surface energy to properly retain ink.
Several processes are known for increasing the surface energy of conventional polymeric films, including corona, flame, and plasma surface treatment. Films subjected to such treatment exhibit a higher surface energy and a higher roughness, which improves various characteristics, including print quality on the film. Typically, such treatments are applied to the films at the time of production, just subsequent to extrusion and in-line prior to converting, and again at a later time just prior to printing. In contrast, conventional film that is not treated at the time of production will typically not satisfactorily accept printing, coating or lamination, even when a later surface treatment is applied.
The effects of the initial surface treatment can diminish over time, e.g. when the films are subject to various environmental conditions, such as high-moisture conditions or after thermoforming. The films can lose sufficient surface energy and become unprintable and uncoatable, and therefore such treatments are often applied again at a later time, e.g. just prior to printing, coating or laminating.
Although the effects of the initial surface treatment diminishes over time or when the film is subject to thermoforming, it is nevertheless necessary for printability of the films. This is because many films, e.g. polyolefin films, are almost untreatable when they set after production, and subsequent surface treatments just prior to printing may not produce satisfactory print adhesion. Therefore, many films require two treatments—one just subsequent to production, and one just prior to printing or other conversion process. Furthermore, these surface treatments often require specialized equipment, which increases the cost of printing on these films.
Accordingly, there exists a need for an improved polymeric material that can be thermoformed, and does not require printing receptive layers or treatments in order to satisfactorily accept and retain printed indicia.